As Donald Trump racked up two more wins this weekend – Louisiana and Kentucky, though both by a slim margin – Mitt Romney is not the only one bemoaning his rise to the top.

Mr. Trump's novel foreign policy proposals, such as forcing Mexico to pay for a massive wall along the southern US border, banning all Muslim immigrants, and most recently, bringing back torture, have prompted anxiety among top world leaders about the possible global reverberations should the real-estate mogul and reality television star become leader of the free world.

Appearing on CBS’ “Face the Nation” Sunday morning, the top Republican presidential contender solidified his stance on torture, vowing to bring back tactics such as waterboarding in order to compete with terrorist groups like the Islamic State.

"You know, it's very tough to beat enemies that don't have any, that don't have any restrictions, all right? We have these massive restrictions. Now, I will always abide by the law, but I would like to have the law expanded," Mr. Trump told host John Dickerson.

"I happen to think that when you're fighting an enemy that chops off heads, I happen to think that we should use something that's stronger than we have right now. Right now, basically, waterboarding is essentially not allowed, as I understand it,” he added. “I would certainly like it to be, at a minimum, at a minimum to allow that."

Meanwhile, Germany’s Vice Chancellor Sigmar Gabriel said in an interview Sunday that Trump is a threat to world peace.

“Whether Donald Trump, Marine le Pen, or Geert Wilders — all these right-wing populists are not only a threat to peace and social cohesion, but also to economic development,” he told German newspaper Welt am Sonntag, alluding to the heads of far-right political parties in France and the Netherlands, respectively.

"If he came to visit our country, I think he would unite us all against him," Mr. Cameron told British parliamentarians.

Last month, lawmakers in Mexico City unanimously passed legislation that urges their federal government to ban the businessman from entering the country. The French Prime Minister, less than a month after the Paris attacks, also expressed disdain for Trump’s discriminatory antics.

But beyond just strained diplomatic ties, some pundits foresee even graver consequences, given the current fragility of Western alliances.

“Right now, we are two or three bad elections away from the end of NATO, the end of the European Union, and maybe the end of the liberal world order as we know it,” writes Anne Applebaum for Slate.

And Trump, she warns, could be the one to tip the balance:

Trump advocates torture, mass deportation, religious discrimination. He brags that he “would not care that much” whether Ukraine were admitted to NATO; he has no interest in NATO and its security guarantees. Of Europe, he has written that “their conflicts are not worth American lives. Pulling back from Europe would save this country millions of dollars annually.”...

Not only is Trump uninterested in America’s alliances, he would be incapable of sustaining them. In practice, both military and economic unions require not the skills of a shady property magnate who “makes deals” but boring negotiations, unsatisfying compromises, and, sometimes, the sacrifice of one’s own national preferences for the greater good. In an era when foreign policy debate has in most Western countries disappeared altogether, replaced by the reality TV of political entertainment, all of these things are much harder to explain and justify to a public that isn’t remotely interested.